The clear 'new make spirit' to emerge from distillation must be matured for at least 3 years in oak casks with a maximum volume of 700 litres in a Scottish bonded warehouse before it can be called Scotch whisky. During this aging period it undergoes profound changes to emerge as an amber whisky. These changes are influenced by the type of oak, previous cask contents and numerous other variables.
Scotch malts are matured in oak casks, for a legal minimum of three years but some are aged for periods as long as 50 years or more. Any 'age statement' on a Scotch whisky bottle or packaging must be numeric and reflect the age of the youngest whisky that comprises that whisky. Sadly, and controversially, regulations do not permit an age breakdown of the individual elements of a blend. At least 60% of the whisky's flavour (and much or all of its colour) comes from the cask.
Two different types of oak wood are typically used in Scotch whiskey maturation, American white oak (Quercus alba) and European oak (Quercus robur). These two species of oak have different compositions of lignin, cellulose, tannins and hemicellulose so will influence the spirit very differently. These differences are amplified by the fact that American oak casks have usually held bourbon while European oak casks tend to be seasoned with sherry and sometimes port or even table wines. The tradition of charring the inside of American oak bourbon casks but only lightly toasting European oak casks also influences maturation.
The casks used to age whisky come in several different shapes and sizes. Sherry casks come in 'Punchon' and 'Butts', both around 500 litres capacity, the latter identifiable as they are fatter. Bourbon 'barrels' are recognisable from sherry casks as their ends have a small indent when compared to the large indent of sherry casks.
A plentiful supply of American oak barrels is ensured due to American law specifying that bourbon must be aged in new oak - meaning that the bourbon distillers are unable to re-use their barrels so are happy to sell them on. These barrels are sometimes broken into staves to reduce shipping costs and reassembled in Scotland, usually using the staves from five standard 190 litre bourbon barrels and reassembled with new ends to make four larger 250 litre hogshead casks.
Whisky matures quicker in smaller casks as the ratio of wood to liquid is greater, but many argue matures more gracefully in larger casks. Diageo in particular favours the use of hogsheads (or 'hoggies') over American standard barrels (ABS). (Note: 'cask' is the generic term while 'barrel' describes a particular type of cask.)
European casks which have been seasoned with sherry in Jerez are usually shipped 'wet', meaning that although the sherry has been emptied the bung will have been replaced so keeping the inside of the cask moist. Contrary to belief, the flavours derived from aging whisky in an ex-sherry cask primarily come from the European oak, not the sherry. Remember it is the oak which originally flavoured and coloured the sherry.
European oak tends to impart rich dried fruit cake, raisin, date and toffee flavours, and has more bitter tannins. Whisky aged in European oak tends to have a dark colour. By contrast, American oak produces lighter coloured whisky and contributes sweeter notes of coconut and vanilla. For more in-depth information on different types of oak, their composition and interaction during maturation.
During construction at the cooperage the inside of the cask is exposed to fire, the heat helping shape the staves while also converting some of the wood's starches into sugars. European oak casks tend to be only lightly toasted while American barrels can be very heavily toasted. The degree of char is usually specified by the distiller who commissions the casks to be make. This thin layer of activated charcoal inside the cask helps remove sulphur compounds and soapy green flavours from the whisky. Charring also breaks down the oak cracking its surface so better allowing the spirit to seep deep into the wood.
Char levels from 1 (light) to 7 (heavy char
Changing weather conditions over the seasons also influence maturation with warm weather raising the temperature of the spirit, causing it, along with the wood's pores to expand allowing the spirit to steep further into the wood. Conversely, cold weather makes the wood pores and the spirit contract. These temperature induced interactions between wood and spirit are known as cycles.
The late 1990's heralded a trend for 'finishing' or 'further aging' malts in casks that have previously been used to age another spirit or wine. Examples include sherry (oloroso, amontillado or fino), Bordeaux (claret & St-Julien), bourbon (including 'Missouri cask'), Rhone wines, rum, cognac, tokay, malaga, port and sauternes. Whiskies aged in this way are often marketed with the type of 'wood finish' indicated on their packaging.
Château Mouton Rothschild Bordeaux wine casks at Bruichladdich
Whether they have previously held sherry, wine, or most commonly bourbon, most of the casks used to age whisky in Scotland are second-hand but despite this the canny Scots use these another three to eight times before they are deemed exhausted and are condemned to become firewood or garden furniture.
A cask may be second-hand but the first time it is filled with Scottish spirit it is termed a 'first-fill' cask. First fill casks are usually filled with grain whisky or lighter new make malt spirit, unless the casks are ex-sherry casks in which case they will immediately be filled with malt. The cask will then be refilled and used again. From the second filling onwards the cask becomes what's termed a 'refill cask' - second refill, third refill etc.
The first fill tends to be for a shorter period, typically 4 to 5 years with subsequent fills being an average of 8 to 12 years per fill (much longer than the three year legal minimum).
The amount of colour or 'tint' a cask imparts to a whisky is measured between fills, more active casks will impart more colour to the whisky while exhausted casks give little colour. The char layer inside the cask extracts sulphur and other unpleasant substances but activity reduces with each fill as the charcoal starts to become saturated with the compounds it has extracted from the whisky. The subsurface below the toasted layer has the sugars and extraction of these is also reduced with subsequent fills.
Most distillers use casks for three to four fills before they are spent but some distillers, most notably Diageo, rejuvenate casks by removing the ends - flaying, scrapping or routing back to new wood and re-toasting/re-charring the insides then replacing the ends. Rejuvenated casks will service another three to four fills and using this method a good oak cask will last for 80 to 100 years - around the same time it takes for an oak tree to reach maturity. Rejuvenated casks tend to produce spicier whiskies.
As the cask is porous, over the years a proportion of the whisky inside (1% to 3% a year) evaporates, allowing some of the volatile compounds escape. This loss is known as the 'angel's share'. Similarly, the pressure deferential due to this evaporation draws Scottish air into the cask. This oxidises the maturing spirit creating acids which react with alcohols to produce esters that impart fruity notes to the whisky. Aromas carried in the air will also influence the whisky, contributing further to the individuality inherent in a distillery's location, particularly those close to the sea.
The strength of the spirit inside the cask will also dramatically affect how the spirit interacts with the wood. Less concentrated spirit hydrated to a lower strength yields better results from aging but obviously requires more casks and more warehouse space. The industry standard filling strength of 63.5% alc./vol. strikes a balance between the cost of casks, warehouse space and the desire to mature the whisky in the best possible conditions.
Traditional dunnage ageing warehouses were low windowless buildings with earth floors covered with spent coke from the distilleries kiln. In these old warehouses casks sit in lines across the floor, two to three casks high, rolled into place using ramps and brute force. Although such warehouses are still in use, most modern warehouses have high racking with casks moved and placed in position on pallets with the aid of forklift trucks.
Traditional dunnage warehouse at Balmenach
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